Hydrocarbon distillation process



HYDROCARBON DISTILLATION PROCESS Oscar R. Buehler, Orange, Tex.,assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, acorporation of Delaware No Drawing. Application March 17, 1954, SerialNo. 416,945

2 Claims. (Cl. 202-57) This invention relates to a process forinhibiting polymerization during the distillation of compositionscontaining unsaturated hydrocarbons.

in the distillation of compositions containing unsaturated hydrocarbons,it is known that polymerization of these compounds is likely to occur.When the hydrocarbon compositions subjected to distillation contain evensmall amounts of diolefin, such as butadiene, isoprene, cyclopentadiene,or the like, there is frequently formed a particularly obnoxious kind ofpolymer which has been designated in the industry as popcorn polymer.This popcorn polymer is obnoxious because it is essentially non-fusibleand insoluble in ordinary solvents. Hence, once the popcorn polymer hasformed, it can be removed from the distillation equipment only with thegreatest difiiculty. This removal, in practice, ordinarily involves acomplete shutdown of the equipment so that the polymer can be removedmanually. Furthermore, once a seed of this polymer has been formed, itpropagates rapidly, despite the presence of ordinary polymerizationinhibitor. Apparently, the initial formation of the popcorn polymer isdependent upon the presence of diolefin.

This popcorn polymer can interfere very seriously with commercialdistillation operations. It adheres tenaciously to heat transfersurfaces, with the result that the rate of heat transfer through theequipment drops off very rapidly. It also forms on the trays or packingin the columns and prevents the normal flow of the liquids and gasestherein. If allowed to propagate for a long enough period of time, thepopcorn polymer can actually result in distortion of rupture of theequipment.

In copending United States patent application of O. R.

Buehler, S. N. 321,711, filed November 20, 1952, now abandoned, it wasdisclosed that inhibitors of a rather ,OPGIflilOilS (describedhereafter) in the purification and distillation of ethylene, for aperiod of six months. Also mentioned in the said patent application S-N. 321,711 was the use of one or more alkyl nitrites in conjunction witha hydrazine-acid mixture or a catechol-ammonia mixture. Also, the UnitedStates patent to E. E. Lewis,

' 2,730,489, discloses the use of alkyl nitrites to prevent popcornpolymer formation and states that a synergistic result is obtained whenthe alkyl nitrite inhibitor 'is used with the catechol type inhibitors.The Lewis application also mentions the use of phenyl hydrazine as aninhibitor component.

In general, the purification processes to which the pres- *entinventio'n'is directed -.and in which it is important to Patented Oct.8,1957

prevent or delay popcorn polymer formation are those in which a solutionof crude ethylene (contaminated with butadiene) in a normally liquidhydrocarbon (e. g. xylene, or a more highly alkylated benzene)ismaintained under elevated pressure (at least 200 pounds per square inch)and at temperatures which generally do not exceed 200 C. Such solutionscan be subjected to conditions of temperature and pressure which releaseselectively an ethylene fraction relatively depleted of higher boilingcontaminants, which upon fractional distillation of the ethylenefraction, yields purified ethylene. Typical mixtures which producepopcorn polymer are the following:

Original After de- After de- Oomponent Solution, ethanizapropanizamolfraotion. mo] tion. moi

tion fraction traction Methane 0. 0032 0. 2822 0. 0069 0. 0703 0. 00320. 2186 0. 3222 0. 0105 0. 1769 0. 2740 0.0518 0. 0048 0. 0076 0. 01750. 0167 0. 0265 0. 0611 Butane-2- 0. 0173 0. 0274 0. 0533 2 Methylbutadiene 0.0182 0. 0288 0. 0066 Cyclopentadiene. 0.0012 0.0019 0. 0044Cyclopentene 0. 0143 0. 0227 0. 0524 0.0200 0. 0317 0. 0732 0. 0239 0.0378 0. 0873 0.0033 0. 0052 0. 0120 0. 0048 0. 0076 0.0175 Cyclohexene0. 0003 0.0005 0. 0011 Benzene 0. 0802 0. 1270 0. 3113 HeavyHydrocarbons, average M. W. 112 0. 0438 0. 0690 0. 1700 Tailstemperature, 0 49 112 1 15 It is an object of this invention to providefurther improvements in processes for distilling compositions comprisingunsaturated hydrocarbons. It is another object of this invention toprovide a polymerization inhibitor capable of preventing or delayingundesirable polymerizations, when carrying out the distillation ofunsaturated hydrocarbons, for longer periods of time than were possibleheretofore. A further object of the invention is to provide an improvedprocess for concentrating or purifying a hydrocarbon containing no morethan 4 carbon atoms from a hydrocarbon composition comprising diolefinsof 3 to 6 carbon atoms, although diolefins of more than 6 carbon atomsmay be present in some variations of this process. A still furtherobject of the invention is to provide an improved process for purifyingethylene wherein a low-boiling hydrocarbon mixture containing saidethylene and also containing a small amount of butadiene is absorbed ina higher-boiling hydrocarbon mixture, and thereafter subjected todistillation in the presence of an effective polymerization inhibitor.Other objects of the invention will appear hereinafter.

In accordance with this invention there is provided a process forminimizing polymer formation when distilling a composition comprising amajor portion of hydro carbons containing no more than six carbon atoms,said composition containing at least 0.01 per cent by weight of adiolefiu having from 3.to 6 carbon atoms, which process comprisescarrying out the distillation of said composition in the presence of aspecific combination of polymerization inhibitors, each constitutingfrom 10 to 1000 parts by weight per million parts of the saidcomposition. The polymerization inhibitors of the present invention arefully described hereinafter.

The process of the present invention is useful in connection with thedistillation, redistillation, concentration and/or purification ofhydrocarbon-containing compositions in which at least a portion of thehydrocarbons comprises a relatively low-molecular weight unsaturatedhydrocarbon. The hydrocarbon mixtures suitable for use in this processare those in which a major portion of the composition, i. e., at least50% by weight, is made up of hydrocarbons containing no more than 6carbon atoms. These relatively low-molecular weight hydrocarbons maybe'paraflins, isoparaffins, olefins, diolefins, alicyclic compounds, oraromatic compounds. In the distillation of compositions of this nature,the major difficulty apparently stems from the presence of one or morediolefins. Hence, the polymerization inhibitors of the present inventionfind the greatest utility in connection with the distillation ofcompositions containing at least a small amount of a diolefin. Ifdesired, the compositions which are subjected to distillation mayconsist almost entirely of diolefins, although the preferred processinvolves the distillation of compositions containing relatively largeamounts of lowmolecular weight paraflins and monoolefins together withonly relatively small amounts of diolefins. The mixture which issubjected to distillation may also comprise nonhydrocarbon portions suchas hydrogen, oxygenated compounds having boiling points close to thehydrocarbons, polymerization inhibitors other than the preferred ones ofthis invention, etc. Preferably, however, the compositions which aresubjected to distillation in accordance with the process of the presentinvention consist almost exclusively of hydrocarbons.

According to one feature of the invention, a mixture consistingprimarily of C2 and C3 saturated and unsaturated hydrocarbons, togetherwith relatively small amounts of C4 and C5 diolefins, is subjected to adistillation operation for the concentration or purification of one ormore of its constituents, notably ethylene. Such a mixture may bederived for instance from the catalytic or non-catalytic cracking ofethane, propane, propylene, butane, or the like. The treatment of such amixture may involve absorbing it in a relatively higher-boilinghydrocarbon absorption medium. The enriched absorption medium may thenbe treated for the stepwise removal of the various hydrocarbons. Forexample, it may be treated initially in a deethanizer, from whichethylene and ethane are removed overhead. The partially strippedabsorption medium may then be passed from the deethanizer to adepropanizer from which propane and propylene are removed overhead. Atleast a portion of the stripped absorption medium that is removed fromthe deethanizer may then be recycled to the absorber. A process of thisgeneral nature is particularly useful for the manufacture of highlypurified ethylene, the ethylene being recovered by fractionation of thegases taken overhead from the deethanizer.

The present invention is based upon the discovery that a specificcombination of inhibitors, namely, (1) a compound of the formula R'NOzin which R is a hydrocarbon radical having from 1 to 8 carbon atoms andhaving an aliphatic carbon atom to which the nitrite group is attached,(2) a catechol, and (3) an aryl hydrazine, exerts an inhibiting effecton popcorn polymer formation and growth in excess of the additiveeffects of the individual inhibitors or any pair thereof. The discoveryof this synergistic inhibition has made it possible to operate ethylenepurification equipment for about one year without closing down theoperation and without difiiculty from popcorn polymer formation.

This synergistic combination of inhibitors can be added at any point inthe distillation unit. The mixed inhibitors can be added at the top ofthe column, at any intermediate portion of the column, or at the bottom,in the pot or calandria. They may be added along with the feed orintroduced into the reflux. They may be added to any one of severalinterconnected distillation columns or to several such columns. They maybe added as the pure compounds, or in solution in a suitable solvent,either as a solid stream or in the form of a mist or dispersion. Whenthe distillation is such that only the very low- ,boiling hydrocarbonssuch as ethane, ethylene, propane or propylene, are being takenoverhead, and the diolefins remain in the liquid phase, satisfactoryresults may be obtained by merely introducing the inhibitors near thebottom of the column or into the pot or calandria. On the other hand, ifone or moreof the diolefins, such as butadiene, isoprene,cyclopentadiene, the piperylenes, or the hexadienes are to be takenoverhead in any appreciable amount, it is highly advisable to introducethe inhibitors at the top of the column or with the reflux or at thecondenser inlet. Furthermore, if any popcorn polymer has had anopportunity to form near the top of the column, it is advantageous tointroduce the inhibitors of the top of the column.

The distillation process of the present invention can be carried outeither batchwise or continuously. Either the heads or the bottoms from agiven column may be passed to a subsequent distillation unit, in whichcase the inhibitors may be carried along and the introduction ofinhibitor at one point in a system can serve to protect a whole seriesof columns or distillation units.

The temperatures and pressures employed are, of course, dependent uponthe particular separation being effected. The synergistic inhibitionmixtures of this invention may be employed satisfactorily indistillation processes operated at either atmospheric, subatmospheric,or superatomspheric pressure.

The concentrations of the inhibitors referred to herein are based uponthe weight of inhibitor present at a particular moment within a givendistillation unit as compared to the total weight of liquid materialpresent in the unit at that time. In order to provide satisfactoryprotection, there should be present at least 0.001 percent of the mixedinhibitors. In some instances, amounts up to 5 percent may be employedor may be required. Usually, however, very small amounts of inhibitorswill provide satisfactory protection and it therefore is generally notnecessary to employ a concentration greater than about 0.10 percent. Inprocesses which are operated continu ously, or where the inhibitor isadded with the feed or the reflux, or where the effluent from one columis passed along to one or more subsequent columns, it will be necessaryto calculate the holdup of a particular distillation unit and the rateof introduction and removal of the inhibitor in order to provide thedesired concentration.

Prior to starting up a given distillation unit, it is sometimes helpfulto circulate through the unit a solution containing a relatively largeconcentration of the inhibitors in a solvent devoid of polymerizableconstituents. In this way, the formation or propagation of popcornpolymer which might otherwise occur during the initial phase of thedistillation, or before the inhibitor reaches a given point, iseffectively prevented.

The superior results obtained when using the polymerization inhibitorsof the present invention are very striking, as shown by the followingexamples.

Example.--In an installation involving the absorption of a crackedhydrocarbon mixture in a higher-boiling hydrocarbon absorption medium (abenzene sponge oil) (475 p. s. i., temperature of overhead stream, 18C.) followed by treatment of the enriched absorption medium (49 C.) in adeethanizer (pressure 500 p. s. i., temperature of overhead stream, 2C., temperature of liquid draw-off from base, 112 C.), and then adepropanizer (pressure 260 p. s. i., temperature of overhead stream 49C.), an inhibitor composition consisting of 260 p. p. m. phenylhydrazine, 350 p. p. m. isoamyl nitrite, and 600 p. p. m. p-tertiarybutyl catechol is introduced into the deethanizer feed. The tails fromthe deethanizer are passed to the middle of the depropanizer. A portionof the tails from the depropanizer, still containing the inhibitors ispassed back to the top of the absorber where it becomes enriched in therelatively lower boiling hydrocarbons. The efliuent from the bottom ofthe absorber is then passed as feed to the middle of the deethanizer,where more of the same inhibitor mixture is added continuously, to makeup for losses in the system and for the portion which leaves the systemat the bottom of the depropanizer. Hence, by introducing the mixture ofinhibitors into the deethanizer, the inhibitors are carried over intothe depropanizer and thence into the absorber, thus providing highlyeffective protection for all three of these distillation units. Thisoperation was continued for about one year without accumulation ofpopcorn polymer in the system. The individual inhibitors, when used inplace of the mixed inhibitors in the same total concentration failed toprevent popcorn polymer formation over a period of six months.

It is noteworthy that not only was the need for frequently replacing orcleaning equipment lessened by the use of the efiective synergisticinhibitors of this invention, but also the rate of production ofpurified ethylene was greatly improved during the time when theequipment was in operation. The specific combination of inhibitors wassuperior to the individual inhibitors, and pairs thereof, but it wasalso superior to combinations of t-butyl catechol and isoamyl nitritewith other effective inhibitors, such as n-butyl p-amino phenol andN,N-disa1icyli-:lene 1,2-diaminopropane. Over a 9.5 month period theproduction rate was about 40% higher due [0 the use of the specificcombination of inhibitors herein set forth, than with multi-componentinhibitors containing only two (i. e. not all three) of the threeinhibitors used in the practice of this invention.

There is a critical range of temperature, above which polymerization ofdiolefin monomer proceeds very rapidly with the preferred concentrationof inhibitors herein disclosed. This critical temperature is generallywithin the range of l0O-200 C. It has been established that, in theprocess illustrated in the above-described example, the deethanizerboil-up temperature, which is substantially the same as the temperatureof the liquid drawn off from the base of the deethanizer (usually heldat 110-120), must not exceed 130"-135 C., if popcorn polymerization isto be suppressed completely.

No satisfactory theory has been advanced to explain the nature andcharacteristics of the popcorn polymer or the dilferences betweenvarious combinations of inhibitors in preventing growth of popcornpolymer. The polymer is a non-fusible, non-soluble material whichappears to be extensively cross-linked. It is not known whether thepresence of air, peroxides, or free radicals has anything to do with theformation or propagation of this polymer. Ordinary free radicalinhibitors of the type which remove peroxides, however, are inetlectiveon preventing its propagation. The inhibitors of the presentinvention'are most effective when employed before any popcorn polymerhas had a chance to form, but even if some of the polymer has formed,these inhibitors are also highly effective in inhibiting furtherpropagation.

As will be readily apparent to those skilled in the art, various changesand modifications may be made in this process without departing from thespirit and scope of this invention.

This application is a continuation-in-part of copending application S.N. 321,711, filed November 20, 1952.

I claim:

1. The method of preventing popcorn polymer formation in a solution ofcrude ethylene in a normally liquid hydrocarbon medium, said ethylenebeing contaminated with butadiene as an impurity, which comprisesmaintaining the said solution under distillation conditions, in thepresence of a mixed synergistic inhibitor consisting of phenylhydrazine, isoamyl nitrite, and p-tertiary butyl catechol, thequantities of each inhibitor being from 10 to 1000 parts per millionparts by weight of said solution, under pressure of at least 260 p. s.i. at a temperature not exceeding the critical temperature above whichpopcorn polymer forms in the mixture, said critical temperature beingwithin the range of C. to 200 C.

2. The method of preventing popcorn polymer formation in a solution ofcrude ethylene in a normally liquid hydrocarbon medium, said ethylenebeing contaminated with butadiene as an impurity, which comprisesmaintaining the said solution under distillation conditions, in thepresence of a mixed synergistic inhibitor comprising 260 parts of phenylhydrazine, 350 parts of p-tertiary butyl catechol, and 600 parts ofisoamyl nitrite, per million parts by weight of said solution, underpressure of at least 260 p. s. i. at a temperature not exceeding thecritical temperature above which popcorn polymer forms in the mixture,said critical temperature being within the range of 100 C. to 200 C.

References Cited in the file of this patent UNITED STATES PATENTS2,361,538 Franz Oct. 31, 1944 2,388,429 McKinnis Nov. 6, 1945 2,407,861Wolk Sept. 17, 1946 2,557,684 Powers June 19, 1951 2,663,415 ChenicekMar. 31, 1953 2,730,489 Lewis et a1. Jan. 10, 1956

1. THE METHOD OF PREVENTING "POPCORN" POLYMER FORMATION IN A SOLUTION OFCRUDE ETHYLENE IN A NORMALLY LIQUID HYDROCARBON MEDIUM, SAID ETHYLENEBEING CONTAMINATED WITH BUTADIENE AS AN IMPURITY, WHICH COMPRISESMAINTAINING THE SAID SOLUTION UNDER DISTILLATION CONDITIONS, IN THEPRESENCE OF A MIXED SYNERGISTIC INHIBITOR CONSISTING OF PHENYLHYDRAZINE, ISOAMYL NITRITE, AND P-TERTIARY BUTYL CATECHOL, THEQUANTITIES OF EACH INHIBITOR BEING FROM 10 TO 1000 PARTS PER MILLIONPARTS BY WEIGHT OF SAID SOLUTION, UNDER PRESSURE OF AT LEAST 260 P.S.I.AT A TEMPERATURE NOT EXCEEDING THE CRITICAL TEMPERATURE ABOVE WHICH"POPCORN" POLYMER FORMS IN THE MIXTURE, SAID CRITICAL TEMPERATURE BEINGWITHIN THE RANGE OF 100*C. TO 200*C.